Hybrid method of rocket propulsion using tetranitromethane



United States Patent Ofiice 3,083,526 Patented Apr. 2, 1963 3,083,526 HYBRID METHOD OF ROCKET PROPULSION USING TETRANITROMETHANE Paul S. Hudson, Bartlesville, kla., assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed Dec. 19, 1958, Ser. No. 783,643 4 Claims. (Cl. 60-354) This invention relates to rocket motors utilizing solid propellants and to an improved method for operating such motors. In one aspect this invention relates to an improved oxidizer for supplying additional oxygen to a fuel-rich solid propellant. In another aspect this invention relates to a method for obtaining improved operational characteristics from a solid rocket propellant.

Recently it has been discovered that superior solid propellant mixtures are obtained comprising a solid oxidizer, such as ammonium nitrate or ammonium perchlorate, and a rubbery binder material, such as a copolymer of butadiene and a vinyl pyridine or other substituted heterocyclic nitrogen base compound, which, after incorporation, is cured by a quaternization reaction or a vulcanization reaction. Solid propellant mixtures of this nature and a process for their production are disclosed and claimed in copending application Serial No. 284,447, filed April 25, 1952, now Patent No. 3,003,861, by W. B. Reynolds and J. E. Pritchard.

The above-described propellant compositions have been found to be superior rocket propellant materials as well as superior gas generating materials, for generation of large volumes of gases at elevated temperatures and pressures, applicable in many industrial systems wherein large bursts of power are required for relatively short periods of time. Such propellant materials, when compounded with a sufiicient amount of the binder so as to render the final composition sutficiently pliable for formation of shaped charges, are deficient in oxygen for complete combustion. The stoichiometric amount of ammonium nitrate is about 94 weight percent with about 6 weight percent hydrocarbon fuel such as is represented by the binder composition. Two-component propellant charges ordinarily contain about 80 to about 90 weight percent ammonium nitrate, so as to avoid problems in the fabrication of shaped charges, and such propellant compositions are therefore considerably deficient in oxygen for complete combustion of the fuel components of the propellant.

It is therefore a principal object of this invention to provide an improved method for obtaining complete combustion of a propellant charge by addition thereto of a novel oxidizer component. It is also an object of this invention to provide a superior oxidizer to supplement the solid oxidizer of the solid propellant charge to be used in a rocket motor. A further object of this invention is the provision of a method for increasing the specific impulse properties of a fuel-rich, solid rocket propellant. Other and further objects and advantages of, this invention will be apparent to one skilled in the art upon study of the disclosure of this invention.

Broadly, the invention comprises the addition of tetranitromethane as a source of supplemental oxygen for the combustion of a fuel-rich solid propellant. It is preferred to use the tetranitromethane without dilution in order to obtain maximum etficiency of the added oxidizer; however, if desired, the tetranitromethane can be diluted with up to about 25 percent of nitrogen dioxide, (N0 or nitrogen tetraoxide, (N 0,), or up to about percent of nitrogen trioxide, (N 0 to lower the melting point of the tetranitromethane. Similarly, the tetranitromethane can be diluted with low boiling nitroalkanes in order to lower the melting point of the tetranitromethane.

Rather than reduce the efficiency of the tetranitromethane as an oxidizer, it is usually preferred to employ a system for introducing the tetranitromethane to the burning surface of the solid propellant by a means which simultaneously adds heat to the te'tranitromethane. Such system is disclosed in copending application Serial No. 502,154, filed April 18, 1955, by H. M. Fox. In the rocket motor described in the copending application, pressure is exerted on the liquid oxidizer by burning a solid charge contained in a flexible bag within the body of the liquid propellant, thus providing simultaneously pressure and heat to the body of liquid. Another rocket motor wherein the oxidizer of this invention is applicable is described in copending application Serial No. 779,022, filed December 8, 1958, now Patent No. 2,984,- 973, by A. F. Stegelman. In this rocket motor a piston exerts pressure upon the oxidizer in a container and causes the liquid oxidizer to be expelled through nozzles in the piston onto the surface of the solid propellant charge and heat can be transmitted through the piston to the liquid oxidizer.

The invention is not limited to the use of a solid propellant containing ammonium nitrate or ammonium perchlorate and a rubbery binder material such as that pre viously described, but is applicable to any solid propellant which has a ratio of fuel to oxidizer greater than stoichiometric. Thus a solid propellant containing ammonium nitrate or ammonium perchlorate and a rubbery binder comprising natural rubber or any of the synthetic rubbers is also applicable. Other fuel-rich solid propellants include ammonium nitrate and an asphalt binder and ammonium nitrate or ammonium perchlorate and various binder materials such as Thiokol polysulfide rubber, polyvinylchloride, etc.; and nitroguanidine, sodium nitrate or potassium nitrate with suitable binder materials such as those set forth above.

A novel solid propellant which is applicable in the practice of the method of this invention is one comprising ammonium nitrate oxidizer, conjugated dieneheterocycli-c nitrogen base copolymer, and a high energy material such as aluminum, lithium, magnesium, or hydrides such as lithium aluminum hydride (LiAlH lithium hydride (LiH), magnesium hydride (MgH etc.

The rubbery materials which are applicable for use as the binder and fuel components of the solid propellant compositions include natural and synthetic rubbery polymers which commonly are characterized by Mooney values (ML4) in the range of about 10 to about 75. The types of rubbery materials which are useful include: natural rubber; and synthetic rubber such as polysulfide rubber, silicone rubber, polybutadiene, butadiene-styrene copolymers, butadiene-acrylonitrile copolymers, and copolymers of butadiene with a copolymerizable heterocyclic base of the pyridine and quinoline series containing a CH ==C group such as Z-rnethyl-5-vinylpyridine. Commonly preferred rubber binders are copolymers prepared by copolymerization of a major amount of a conjugated diene such as 1,3-butadiene with a minor amount of a copolymerizable monomer containing the CH =C group such as Z-methyl-5-vinylpyridine. The /10 copolymer of 1,3-butadiene/2-methyl-S-vinylpyridine has been found to be particularly suitable and is frequently preferred because of the favorable physical characteristics of the resulting propellant composition when such copolymer is used as the binder component.

These rubber polymers are compounded, as is wellknown in the rubber art, with a reinforcing filler such as carbon black, with antioxidants, with plasticizers, and with curing agents such as vulcanizing or quaternizing agents.

The plasticizers include the known rubber plasticizers such as compounds including dioctyl phthalate, benzophenone, amylbiphenyl, di(bu-toxyethoxyethoxy) methane, trioctyl phosphate, tricresol phosphate, and liquid polymers of 1,3-butadiene. These plasticizers are commonly used in amounts of to about 25 parts by weight per 100 parts by weight of rubber. The plasticizer is generally used in amounts sufiicient to soften the rubber so as to facilitate incorporation of the oxidizer therein.

(hiring agents for use with rubbery copolymers are well-known in the art and include sulfur, sulfur-liberating materials, such as thiuram disulfides, polysulfides, alkylphenol sulfides and N,N-dithioamines. Nonsulfur curatives are also known. Accelerator-activators are frequently used in conjunction with the curing agents and examples of such compounds include lead oxide, zinc oxide, magnesium oxide, mixtures of magnesium oxide and carbon, lead carbonate, hydrated lime, lead silicate, dibutyl ammonium oleate, oleic acid, dibenzylamine, linseed oil, fatty acids, rosin acids, triethanolamine, zinc stearate, and the like. When the copolymers include a heterocyclic nitrogen base, as herein-before described, as one of the monomers, quaternizing agents are sometimes used as the curing agent. These quaternizing agents include various alkyl halides such as methyl iodide, ethyl iodide, hexyl iodide, octyl bromide; various alkylene halides such as propylene chlorides; various substituted alkanes which contain at least one hydrogen attached to the carbon atom attached to a halogen such as chloroform, 1,3-dichloropropane, ethylene chlorohydrin; various substituted aromatic compounds such as picryl chloride, benzyl chloride, benzene sulfonyl chloride, benzal chloride, benzotrichloride, methylbenzene sulfonate; and various polyhalogenated cycloalkanes such as hexachlorocyclopentadiene, and hexachloro-p-xylene.

Antioxidants commonly used in rubber compounding can be employed and these include hydroquinone, monobenzyl ether, phenyl-beta-naphthalamine, polymerized trimethyldihydroquinoline, heptylated diphenylamine, mono esters of salicyclic acid, hexachloronaphthalene, hydrocarbon waxes, etc.

The above list of rubbery polymers, plasticizers, and other compounding ingredients is illustrative but is not exhaustive of the compounds which can be employed and is not to be considered as limiting. The rubbery polymer, together with such compounding ingredients as are required or desired to obtain a specific result comprise that which is designated as the binder component of the propellant composition. 'In the practice of the present invention, the binder is commonly used in amounts of to parts by weight for correspondingly 84 to 94 parts by weight of one or more solid oxidizers and about one part of burning rate catalyst per 100 parts by weight of propellant.

A general formulation for a binder composition is given in Table I below.

Table I Composition: Parts by weight Rubbery copolymer (as hereinbefore described) 100 Reinforcing agent 0100 Plasticizer 025 Antioxidant O3 Sulfur 0-2 Accelerator-activator 05 Alkyl halide 0-5 Suitable oxidizers include solid, inorganic oxidizing salts such as one or more of the ammonium and alkali metal salts of nitric and perchloric acids including ammonium nitrate, potassium nitrate, sodium nitrate, lithium nitrate, ammonium perchlorate, potassium perchlorate, and the like. Phase-stabilized ammonium nitrate, which comprises about 6 to 15 percent by weight of potassium nitrate in admixture with the ammonium nitrate, is frequently a preferred oxidizer because such stabilized ammonium nitrate is less sensitive to temperature changes. These inorganic oxidizing salts can be used with minor amounts of organic oxidizers and modifiers.

Burning rate catalysts include iron oxide, ammonium dichromate, milori blue, and other complex metal cyanides including the complex cyanides of iron and nickel, such as ferric ferrocyanide and ferrous ferricyanide and are used in amounts of about 0.1 to about 10 percent by weight of the solid propellant composition. The burning rate catalyst can be omitted if desired.

Solid, high-energy fuels, such as powdered metals including magnesium, boron, aluminum and iron, are also sometimes used in amounts up to about 10 percent by weight of the solid propellant composition.

A specific binder composition is shown in the following Table II.

Table II Composition: Parts by weight /10 butadiene-methylvinylpyridine copolymer Carbon black 20 Benzophenone 10 Pentaryl A 10 Tert-butylsulfenyldimethyldithiocarbamate 1 Flexamine 3 Zinc oxide 3 Aerosol OT 1 Sulfur 0.75

The above binder composition was combined with ammonium nitrate and milori blue to make a solid propellant composition having 80 parts by weight ammonium nitrate, 20 parts by weight binder composition and 2 parts by weight milori blue. This propellant composition displays a specific impulse of lb.-sec./lb. for the propellant alone and this is increased to 260 lb.-sec./ lb. for the maximum performance achieved with a ratio of tetranitromethane to propellant of about 0.52. This indicates that for maximum performance about one-half pound of tetranitromethane will be required for each pound of solid propellant.

Another solid propellant composition is shown in the following Table III.

Table III Composition: Parts by weight Polypropylene glycol 1 17.84 Quadrol Z 0.26 Hylene T 3 1.90 Ammonium perchlorate (210 mu) 56.00 Ammonium perchlorate (l8 mu) 24.00 Ferric acetonylacetonate 0.025

Elastomer grade marketed by Union Carbide and Carbon Company and designated as Niox 2025.

2 ,N,N,N-tetrakis (2-hydroxypropyl) ethylenediamine.

'2,et-tolyienediisoeyanate marketed by Du Pont Company.

The table shows that the propellant composition contains 80 weight percent oxidizer and 20 weight percent fuel-binder and is therefore fuel-rich. One part by weight of tetranitromethane is required for 4 parts by weight of propellant for optimum combustion and the increase in specific impulse is comparable to that with the propellant composition containing ammonium nitrate oxidizer.

Tetranitromethane, when used as the auxiliary source of oxygen for the combustion of a fuel-rich solid propellant, increases the specific impulse of the propellant to an unexpected high level. Thus, tetranitromethane increases the specific impulse of the ammonium nitrate containing propellant from 195 lb.-sec./lb. to 260 lb.- sec./lb., whereas nitric acid increases the specific impulse of this propellant to 245 lb.-sec./ lb. For the purpose of augmenting the supply of oxygen in a fuel-rich solid pro pellant, tetranitromethane is superior to nitric acid, hydrogen peroxide and other liquid oxidizers. Tetranitromethane produces an unexpected increase in the impulse characterics of a solid propellant wherein a hydrocarbon or hydrocarbon derivatives is utilized as the binder component of the propellant charge.

For maximum specific impulse it is desirable to supply tetranitrometh-ane to a fuel-rich solid propellant so that a substantially stoichiometric balance of oxygen to fuel results; however, it may be desirable to supply tetranitromethane to the solid propellant in an amount which is lesser or greater than the stoichiometric amount. Usually the amount of tetranitromethane will be in the range of 0.2 to 0.8 part by weight per part by Weight of solid propellant.

Reasonable variations and modifications are possible within the scope of the present disclosure without departing from the spirit and scope of the invention.

That which is claimed is:

1. In the method of developing thrust which comprises ejecting from a rocket motor combustion chamber the gaseous products produced by combustion of a solid, rocket propellant comprising about 50 to about 90 parts by weight of a solid, inorganic oxidizing salt, about 8 to about 48 parts by weight of a rubber binder, and about 1 to about 2 parts by weight of a combustion catalyst, the improvement which comprises injecting tetranitromethane into said combustion chamber in an amount of 0.2 to 0.8 part by weight per part by weight of solid propellant and simultaneously adding heat to said tetranitromethane.

2. In the method of developing thrust which comprises ejecting from a rocket motor combustion chamber the gaseous products produced by combustion of a solid, rocket propellant comprising about 50 to about 90 parts by weight of ammonium nitrate, about 1 to about 20 parts by weight of a metal selected from the group consisting of aluminum, magnesium, boron, beryllium, and lithium, about 8 to about 48 parts by weight of butadiene-methylvinylpyridine copolymer, and about 1 to about 2 parts by weight milori blue, the improvement which comprises injecting into said combustion chamber liquid tetranitromethane in an amount of 0.2 to 0.8 part by weight per part by weight of solid propellant and simultaneously adding heat to said tetranit-romethane.

3. The method of increasing the specific impulse developed by ejecting froim a rocket motor combustion chamber the gaseous products of combustion of a solid rocket propellant comprising a solid inorganic oxidizing salt and a rubber binder wherein the solid inorganic oxidizing salt is present in an amount of about to Weight percent, which method comprises injecting liquid tetranitromethane into said combustion chamber in an amount of about 0.2 to 0.8 part by Weight per part by weight of solid rocket propellant.

4. The method of increasing the specific impulse developed by ejecting from a rocket motor combustion chamber the gaseous products of combustion of a solid rocket propellant comprising about 80 parts by weight ammonium nitrate, about 20 parts by weight of a butadiene-methyl vinylpyridine copolymer binder and about 2 parts by weight of milori blue, which method comprises injecting about 0.5 part by weight of liquid tetranitromethane per part of solid rocket propellant into said combustion chamber.

References Cited in the file of this patent UNITED STATES PATENTS 2,791,883 Moore et al. May 14, 1957 2,877,504 Fox Mar. 17, 1959 OTHER REFERENCES Zaehringer: Solid Propellant Rockets, American Rocket 00., Box 1112, Wyandotte, Michigan (1955), pp. 5, 6 and 140.

Chem. and Eng. News, Oct. 7, 1957, pp. 62-3.

Leonard: Journal of the American Rocket Society, No. 72, December 1947, page 12.

Tschinkel: Industrial and Engineering Chemistry, vol. 48, No. 4, pp. 732-5. 

1. IN THE METHOD OF DEVELOPING THRUST WHICH COMPRISES EJECTING FROM A ROCKET MOTOR COMBUSTION CHAMBER IN GASEOUS PRODUCTS PRODUCED BY COMBUSTION OF A SOLID, ROCKET PROPELLANT COMPRISING ABOUT 50 TO ABOUT 90 PARTS BY WEIGHT OF A SOLID, INORGANIC OXIDIZING SALT, ABOUT 8 TO ABOUT 48 PARTS BY WEIGHT OF A RUBBER BINDER, AND ABOUT 1 TO ABOUT 2 PARTS BY WEIGHT OF A COMBUSTION CATALYST, THE IMPROVEMENT WHICH COMPRISES INJECTING TETRANITROMETHANE INTO SAID COMBUSTION CHAMBER IN AN AMOUNT OF 0.2 TO 0.8 PART BY WEIGHT PER PART BY WEIGHT OF SOLID PROPELLANT AND SIMULTANEOUSLY ADDING HEAT TO SAID TETRANITROMETHANE. 